Rated 4 out of 5 stars
Love! Love! Love! this extension. It's the reason I use Firefox.
However, it's nearly unusable after about a day of use and 20-30 tabs open because it causes Firefox to become very sluggish. Frequent pauses, choppy scrolling, and beach balls (on Mac) get worse and worse until browser is restarted.
I have verified (by systematically eliminating other extensions) that this extension is the culprit.
Would give 5 stars (6 if I could) except for this issue.